General Discussion of Liquid Jetting
It is well-known that a stable liquid jet is formed by forcing a liquid of sufficient viscosity through an orifice—(Edmund, 2006) Macroscopic liquid jets are observed in nature when a pressurized vessel containing a liquid of sufficiently low viscosity is punctured. The flow from the orifice created by the puncture produces a liquid jet with a flow rate that is dependent on the orifice diameter and the pressure within the vessel. Common examples of moderate viscosity jets are honey or oil flowing from an opening. It is also commonly known that jets of medium to low-viscosity liquids, i.e. water, ethylene glycol, and isopropanol, have inherent instabilities and can break up into droplets at varying distances from the jet orifice. Many studies however have been performed to confirm the stable propagation of medium to low-viscosity jets before filament breakup—(Habibi, 2010), (Edmund, 2006). Habibi has performed experimental studies of buckling of stable filaments of silicon oil ejected from an orifice: The silicon oil filaments propagated along distances as great as 60 cm. Edmund has studied the stability of a jet of a viscoelastic liquid formed using hydrodynamic focusing. (Takahashi, 1969) has reported liquid jets of water in air propagating to distances as large as 20 cm before breakup.
The Microfluidic Liquid Jet printing concept was inspired by the observation of stable jets of liquid commonly seen in nature. Examples of microfluidic liquid transport and microfluidic liquid jets are prevalent in nature. Indeed, over 40,000 known species of spiders exist, most of which are classified as web-spinning. Spider silk fibers are spun from pressurized abdominal sacs containing a polymeric solution. The jet dries in-flight to diameters of approximately 2.5 to 4.0 microns, and is used to make intricate patterned webs. The present invention produces a stable co-axial liquid jet by maintaining a constant pressure within a microfluidic flow cell used to form the co-axial liquid distribution.
General Description of Hydrodynamic Focusing
In hydrodynamic focusing, an annular distribution of a core liquid and a sheath liquid is forced through a channel or nozzle, with the core liquid being stretched into a thin filament as the liquids accelerate through the constriction. The width of the core filament is a function of the ratio of the core and sheath flow rates. In hydrodynamic focusing, the diameter of the core liquid is proportional to the fractional volume occupied by the core liquid. As the ratio of the core liquid flow rate to the sheath liquid flow rate is decreased, the volume occupied by the core liquid decreases, and thus reduces the diameter of the core liquid filament. In hydrodynamic focusing applications, the core liquid is stretched into a filament with a width as small as 1 micron.
Hydrodynamic Focusing for Direct Printing Applications
In a Direct Printing technique, a liquid is deposited onto a substrate without the use of masks or lithographic techniques. The present invention uses hydrodynamic focusing to form a thin filament of ink surrounded by a sacrificial sheath liquid. In one application of hydrodynamic focusing to direct printing, two miscible liquids with limited diffusivity are used to obtain focusing of the core liquid. The radius of the core liquid is proportional to the ratio of the core and sheath flow rates, and in application of the present invention, can be varied from approximately 1 micron to 1000 microns.